1. Field
Example embodiments relate to a multi-layered bipolar field-effect transistor and a method of manufacturing the same, and, more particularly, to a multi-layered bipolar field-effect transistor that includes a gate electrode, a gate insulating layer, a hole transport layer, an electron transport layer, a source electrode and a drain electrode, in which an intermediate separating layer is formed between the electron transport layer and hole transport layer, so that the function of each carrier transport channel can be sufficiently realized, and various devices can be manufactured through a solution process without damaging any of the layers, thereby improving processability, and to a method of manufacturing the same.
2. Description of the Related Art
Generally, an organic field-effect transistor exhibits excellent performance as a constituent of cheap, flexible and portable electronic devices, such as smart cards, electronic ID tags, active matrix displays, and the like. Recently, as the demand for small and light devices has increased, an organic bipolar field-effect transistor, which can be easily designed and manufactured by integrating a P-type channel and an N-type channel in one device, has attracted considerable attention.
A bipolar field-effect transistor is a device that is manufactured using bipolar technology. Since the bipolar field-effect transistor has the ability to prevent the destruction of devices by nonuniform current occurring in a pure bipolar device, and has high current capacity, very low ON-resistance, high switching speed and a negative temperature coefficient, the bipolar field-effect transistor is receiving a lot of attention in the high-voltage and high-speed application fields. However, since organic materials generally exhibit very high hole mobility and electron mobility, it is very difficult to manufacture an organic bipolar field-effect transistor.
In order to realize an organic bipolar field-effect transistor, research on materials, in which electrons and holes are simultaneously transported to two metals, constituting a source electrode and a gate electrode, and thus holes are injected into a hole injection electrode composed of a metal having a high work function, and electrons are injected into an electron injection electrode composed of a metal having a low work function, under an opposite gate voltage bias, is being conducted. Further, as methods of realizing an organic bipolar field-effect transistor, attempts to use a mixture of an electron transport material and a hole transport material as an active layer material in a channel have been made, and bipolar field-effect transistors having a two-layered structure including a hole transport layer and an electron transport layer have been disclosed.
The method of realizing an organic bipolar field-effect transistor using a mixture of an electron transport material and a hole transport material is advantageous in that a channel can be formed in one step by treating a semiconductor mixture solution. However, in this method, it is difficult to obtain a continuous network of two components, that is, an electron transport material and a hole transport material, in a limited space adjacent to the interface between the space and a gate electrode because two-dimensional percolation must be conducted under the condition such that the volume fraction of the solution is above 50%. Moreover, in this method, the mobility of electrons or holes in complicated channels located in a phase-separated two-component mixture is remarkably decreased, compared to the state in which each of the components is pure. Further, in this method, there is a problem in that, when holes and electrons coexist in the same channel, non-FET behavior can occur when low gate voltage is applied thereto. It is inferred that this non-FET behavior occurs because electrons and holes are recombined in the interface between a P-type material and an N-type material by Coulomb force. Accordingly, the bipolar FET, manufactured using the mixture of the electron transport material and the hole transport material, typically realizes a low current ON/OFF ratio.
Meanwhile, a hetero-structured field-effect transistor having an independently-separated hole transport channel and electron transport channel was also disclosed. However, in the hetero-structured FET, there is a problem in that the realization of the real behavior of the P-channel and N-channel at low gate voltages is prevented by the interaction between carriers located in the interface between the two layers. Furthermore, there is a problem in that, when the two layers are fabricated through a solution process, a first layer, which is previously formed beneath a second layer, can be damaged by layering the second layer on the first layer. These problems can be minimized using two insoluble materials, but, in most of the attempts to manufacture a two-layered bipolar FET, cannot be overcome because the two-layered bipolar FET is manufactured by thermally depositing small molecules under high vacuum conditions.